Add an initial .mailmap

This is to fix the output of `git shortlog` and `git check-mailmap`.

I only added names and addresses for developers with a very large
number of commits. It could be improved to add more.

.mailmap

+#
+# This list is used by git-shortlog to make contributions from the
+# same person appearing to be so.
+#
+
+Achilleas Pipinellis <axilleas@axilleas.me> <axilleas@archlinux.gr>
+Achilleas Pipinellis <axilleas@axilleas.me> <axilleas@users.noreply.github.com>
+Dmitriy Zaporozhets <dzaporozhets@gitlab.com> <dmitriy.zaporozhets@gmail.com>
+Dmitriy Zaporozhets <dzaporozhets@gitlab.com> <dzaporozhets@sphereconsultinginc.com>
+Douwe Maan <douwe@gitlab.com> <douwe@selenight.nl>
+Douwe Maan <douwe@gitlab.com> <me@douwe.me>
+Grzegorz Bizon <grzegorz@gitlab.com> <grzegorz.bizon@ntsn.pl>
+Grzegorz Bizon <grzegorz@gitlab.com> <grzesiek.bizon@gmail.com>
+Jacob Vosmaer <jacob@gitlab.com> <contact@jacobvosmaer.nl>
+Jacob Vosmaer <jacob@gitlab.com> Jacob Vosmaer (GitLab) <jacob@gitlab.com>
+Jacob Schatz <jschatz@gitlab.com> <jacobschatz@Jacobs-MacBook-Pro.local>
+Jacob Schatz <jschatz@gitlab.com> <jacobschatz@Jacobs-MBP.fios-router.home>
+Jacob Schatz <jschatz@gitlab.com> <jschatz1@gmail.com>
+James Lopez <james@jameslopez.es> <james@gitlab.com>
+James Lopez <james@jameslopez.es> <james.lopez@vodafone.com>
+Kamil Trzciński <kamil@gitlab.com> <ayufan@ayufan.eu>
+Marin Jankovski <maxlazio@gmail.com> <marin@gitlab.com>
+Phil Hughes <me@iamphill.com> <theephil@gmail.com>
+Rémy Coutable <remy@rymai.me> <remy@gitlab.com>
+Robert Schilling <rschilling@student.tugraz.at> <Razer6@users.noreply.github.com>
+Robert Schilling <rschilling@student.tugraz.at> <schilling.ro@gmail.com>
+Robert Speicher <robert@gitlab.com> <rspeicher@gmail.com>
+Stan Hu <stanhu@gmail.com> <stanhu@alum.mit.edu>
+Stan Hu <stanhu@gmail.com> <stanhu@packetzoom.com>
+Stan Hu <stanhu@gmail.com> <stanhu@users.noreply.github.com>
+Stan Hu <stanhu@gmail.com> stanhu <stanhu@gmail.com>
+Sytse Sijbrandij <sytse@gitlab.com> <sytse+admin@gitlab.com>
+Sytse Sijbrandij <sytse@gitlab.com> <sytse@dosire.com>
+Sytse Sijbrandij <sytse@gitlab.com> <sytses@gmail.com>
+Sytse Sijbrandij <sytse@gitlab.com> dosire <sytse@gitlab.com>